1. Field of the Invention
This invention relates to photographic system having image deflecting means and, more particularly, to a photographic system having image deflecting means in which by using a variable vertical angle prism provided in a lens system, an image blur caused by camera shake, etc. is compensated for to stabilize a photographic image, and which is suited for cameras for photography or video cameras.
2. Description of the Related Art
When shooting from a running car, etc., a shake propagates into the photographic system. This leads to blur an image to be formed. There have been many proposals for a photographic system having image deflecting means in the form of a parallel flat plate or a variable vertical angle prism arranged in the optical system to compensate for that blur of the image.
For example, Japanese Patent Publication No. Sho 56-21133 discloses a photographic system utilizing the variable vertical angle prism to compensate for the image blur. In the Publication, a space between two flat glass plates is filled with liquid or transparent elastic material in seal and the angle of intersection of these glass plates is made to vary for the purpose of compensating for the blur of the image.
Furthermore, that Publication proposes another photographic system in which a plano-convex lens and a plano-concave lens are mated at their spherical surfaces in slidable relation to make variable the angle of intersection of their plane surfaces.
In general, with the variable vertical angle prism for compensating for the image blur arranged in front, or in the interior, of the lens system, when the vertical angle of that prism increases to a considerably large value, a decentering lateral chromatic aberration due to the chromatic dispersion of the prism is produced in the deflected image.
This function is illustrated in FIGS. 14(A) and 14(B). P is a variable vertical angle prism. When the vertical angle is varied as shown in FIG. 14(A), the spectral g-line is caused to deflect greater than the d-line. The use of such a variable vertical angle prism P in combination with the master lens M as shown in FIG. 14(B) for the purpose of deflecting the image, therefore, leads to produce a decentering lateral chromatic aberration .DELTA.(.DELTA.Y).
This decentering lateral chromatic aberration occurs almost by the same magnitude and in one and the same direction over the entire area of the image plane from its center to its margin, thus becoming a cause of lowering the contrast and of producing stain of colors. Since the decentering lateral chromatic aberration differs from the ordinary lateral chromatic aberration in that it appears even in such a central portion of the image plane that the image of the object of principal photographic interest often occupies, it is a serious cause of lowering image quality.
To overcome this problem, U.S. Pat. No. 3,514,192 and Japanese Patent Publication No. Sho 57-7416 propose the use of two variable vertical angle prisms of different dispersion arranged to be driven with maintenance of a constant angle ratio so as to satisfy the achromatic condition, thus providing photographic systems capable of reducing a decentering lateral chromatic aberration.
However, this method, because of the necessity of the two variable vertical angle prisms, tends to increase the axial thickness of a deflecting element and has another tendency that the structure of a mechanism for driving the two prisms while maintaining constant the angle ratio becomes relatively complicated.
Also, Japanese Laid-Open Patent Application No. Sho 61-223819 proposes the use of one variable vertical angle prism in combination with a cemented lens whose refractive power is equal to almost zero and which is arranged behind the photographic system. By driving the cemented lens to decenter in a direction perpendicular to the optical axis in synchronism with a motion of the variable vertical angle prism, despite the use of the variable vertical angle prism type, the decentering lateral chromatic aberration is corrected.
In addition, in Japanese Patent Publication Sho 56-40805, by utilizing the rotative slide type of the plano-convex and plano-concave lenses, a prism is formed to have a substantially variable vertical angle. Thus, the accidental displacement of the photographic system, or the blur of the image, is compensated for.
Meanwhile, in the image stabilizing optical system of the type using the function of the variable vertical angle prism, not only the decentering lateral chromatic aberration, but also a decentering distortion come to produce as well.
FIGS. 17(A), 17(B) and FIG. 18 are diagrams illustrating the state of production of a decentering distortion caused by the variable vertical angle prism.
FIG. 17(A) is the initial state where the two surfaces of the variable vertical angle prism are parallel to each other, rays of light o, a and b being principal rays of light beams which are to focus at an image plane center o', off-axial points a' and b', respectively.
FIG. 17(B) and FIG. 18 show a state of the principal rays o, a and b when the variable vertical angle prism has a vertical angle A. As the second surface of the variable vertical angle prism inclines by an angle A.degree., a ray of light coming from an object enters the first surface of the variable vertical angle prism having the vertical angle A at an angle of incidence .theta.. It is then refracted by the first and second surfaces and exits from the variable vertical angle prism at an angle .theta.p with the optical axis according to the following formulae: EQU .theta..sub.1 =sin.sup.31 1 (sin .theta./Np) EQU .theta..sub.2 =.theta..sub.1 +A EQU .theta..sub.2 '=sin.sup.31 1 (Np.multidot.sin .theta..sub.2) EQU .theta.p=.theta..sub.2 '-A
where .theta..sub.1 is the angle of refraction at the first surface of the variable vertical angle prism, .theta..sub.2 is the angle of incidence on the second surface, .theta..sub.2 ' is its angle of emergence, .theta.p is the angle that the exiting ray makes with the optical axis, and Np is the refractive index of the medium of the prism. The principal ray having the angle .theta.p with respect to the optical axis, when produced, arrives on an image plane at a height y' expressed by the formula: y'=f.multidot.tan.theta.p.
Therefore, if the angles of deflection .delta.(.delta.=.theta.p-.theta.) of the principal rays at the second surface are the same, the image, while keeping the original shape, is deflected on the image plane. In fact, however, as their angles of incidence on the variable vertical angle prism differ from one another, the principal rays are deflected at different angles. Hence, all the points o', a' and b' on the image plane behave in different ways from one another as shown in FIG. 17(B). This can be evaluated quantitatively in terms of a ratio of a minute angle dA by which the vertical angle of the variable vertical angle prism changes from the initial state to the resultant change d.theta.p of the angle of emergence of a ray with respect to the optical axis, i.e., the sensitivity, as a function of the angle of incidence .theta. on the variable vertical angle prism as follows: ##EQU1## This equation represents that the larger the absolute value of the angle of incidence .theta., the larger the positive value of the sensitivity d.theta.p/dA becomes.
In the case of the on-axial ray o, because its angle of incidence on the prism surface is small, the angle of deflection .delta.(.delta.=.theta.p-.theta.) resulting from the change of the vertical angle, i.e., the angle of incidence on the second surface of the prism, to the value A is in proportional relation to the vertical angle A, that is, represented by EQU .delta..apprxeq.(Np-1)A
In a case where the angle of incidence on the second surface is relatively large like the off-axial ray a, on the other hand, even if the vertical angle is similarly changed to the value A, the absolute value of the angle of deflection becomes larger than that found by the above-cited equation: .delta.=(Np-1)A.
For example, the off-axial ray a, when the vertical angle has changed in the direction shown in FIG. 18 to the value A, has its angle of incidence .theta..sub.2 on the second surface of the variable vertical angle prism change in a direction to be larger than when in the initial state, so that the angle .theta.p that the exiting ray makes with the optical axis gets larger than .theta.+.delta.. On the image plane, therefore, a distortion in an "over" direction appears as shown by an arrow a.sub.A ' in FIG. 17(B).
Another off-axial ray b, when the vertical angle has become the value A, has its angle of incidence .theta..sub.2 on the second surface of the variable vertical angle prism change in a direction to take a smaller absolute value than that when in the initial state, so that the angle .theta.p that the exiting ray makes with the optical axis gets smaller in the absolute value than .theta.+.delta.. On the image plane, therefore, the distortion is formed also in an "over" direction as shown by an arrow b.sub.A ' in FIG. 17(B).
When such decentering distortion is present, an object to be photographed as shown in FIG. 19(A) is imaged in a distorted form as shown by solid lines in FIG. 19(B), where dashed lines represent an ideal image in the case of lack of the decentering distortion.
Accordingly, in the vibration proof optical system using the variable vertical angle prism for compensating for the image blur resulting from camera-shake by deflecting the image in the reverse direction, as has been described above, if the decentering distortion is present, the amount of movement of the image at the central point in the image plane differs from those at the marginal points. Even though the compensation for the image blur is perfect in the center of the image frame, it is in the marginal zone that the image would diffuse. FIG. 19(C) is a result of compensation for the blur of the image of the object of FIG. 19(A) by the vibration-proof optical system having the decentering distortion.